This invention concerns data acquisition systems where the data sources are spatially spread over an area and it is necessary to collect, manipulate, and display the data for monitor or control purposes at a central location. Specifically, this invention addresses the practical issues related to wiring from the sensors to the central location and providing power to operate the remote electronics and sensors used for measurement.
The system wiring and remote power issues are common in large areas or buildings where the sensors are distant and electrical power may not be readily available. Examples of this are industrial process control, environmental monitoring, and proprietary alarm systems. In these cases, the cost and complexity of system wiring and sensor power are primary project issues.
Present day computing power allows manipulation, organization, and display of large quantities of data at a central location. Because of the ease and efficiency of manipulating and displaying collected data, there is a growing need to collect and process ever increasing amounts. If the data to be collected is spatially spread and randomly located some distance from the central location, then the overriding issue becomes how to collect and get the data to the central location.
There are various methods of accomplishing the collection task that range from individually wired sensors to the central location to use of data busses (eg: RS232, 485) to reduce the wiring requirement. All of these require either intensive wiring, special types of wire (cable or 4 conductor), special routing, or individual sensor wiring to a network nodal point where the sensors are clustered for individual wiring to the nodal point for bus interfacing. Many of these require external power at the sensors, nodal points, or both. This can require AC availability, DC power supplies, and possibly battery backup throughout the data collection network.
Other hybrid methods of accomplishing data collection exist that power the nodal points by simply adding another wire for ground and one for voltage which results in a four wire network. Since one of the wires is ground, two are used for signaling and share a common ground with the power return. This adaptation has the limitation of adding signal and power currents in a common ground that restricts practical use because of electrical losses. Individual nodal or remote unit power current is much larger than signal current and the effect of these losses is further exacerbated at the central unit where the summation of all network power loops produce the highest offsets to signaling levels.
As a practical issue, given that the sensors are spatially spread and the object is to simplify and reduce wiring, it is important that the network not require special wire type or routing rules and that the cluster level is small. Although such systems allow a large number of sensors, the wiring efficiency is quickly lost due to the nature of random sensor location using special routing with high cluster levels. Clearly, a lower level of clustering, for example 1 or 2, with self contained power and no routing rules is more efficient than a higher cluster level, for example 8 or 16, that requires external power and special routing. Further, system failures such as loss of a clustering or nodal device have a larger system effect. Additional complications surface if the central location is moved or if central location redundancy is a reliability requirement.
Multiplex systems all require remote random location of nodal points or remote units as described by this invention to reduce wiring. However, this implies different environments at the nodal points or remote units that will cause measurement variations from the different locations for the same input voltage. Examples of these error sources are climatic environment, operating voltages, linearity differences, and aging. A practical system must incorporate a means of routinely calibrating measurements at the point where they are taken in order to calibrate measurements from that point for a satisfactory degree of system measurement accuracy.
Synchronization loss can occur from network transients caused by power surges, intermittents, and static electricity among other things. If synchronization is lost, the system must recover to avoid network contention and report correct data. This ability requires the use of a synchronization technique that is integral to each sequence of the network routine and global in that it overrides all network activity and configures all network devices to a known starting state.